Recent advances in high power fiber lasers have shown the utility of fiber lasers for high power industrial applications such as metal welding and cutting and also for military applications. Unlike many industrial applications, military applications generally involve propagating a beam over a long distance, which requires good laser beam quality in addition to high power. High beam quality in fiber lasers can be achieved with single-mode (SM) fiber lasers. However, one of the major hurdles to power-scaling SM fiber lasers to the 10-kilowatt level (generally necessary for military applications) has been the difficulty of coupling high power pump light beams into the SM active fiber laser.
The current state of the art uses diode pump light beams for coupling into a doped fiber of a laser. These coupling methods fall into two categories: end-pumping and side-pumping. In end-pumping methods, pump light beam is coupled in free-space through the end facets of a laser fiber. A limitation of this method is that pump light beams have a limited access area to the laser fiber because of the small size of the fiber facets. The fiber facets are normally less than 0.4 mm in diameter. The resulting necessary logistics of diode pump light beam shaping and coupling adds to the complexity of system and reduces system reliability and stability. Ultimately, the coupled power is limited by damage to the anti-reflection coatings of the fiber facets.
In side-pumping methods, pump light beam is coupled through the sides of the fiber. For example, U.S. Pat. No. 5,854,865 issued to Goldberg discloses a fiber amplifier side-pumped by a V-shaped notch cut into the cladding of the doped amplifier fiber. Pump light beam is then reflected off the tilted planes of the notch and directed along the core of the fiber. In U.S. Pat. No. 6,625,345 issued to Hollister, a prism in optical contact with the side of the fiber is used to couple pump light beam into the fiber. In U.S. Pat. No. 5,999,673 issued to Samartsev et al., coupling between a multimode pump fiber pigtail and a double-clad optical fiber is disclosed. The pigtail end of the multimode fiber is tapered and fused to the cladding of the doped optical fiber.
None of the prior art methods combine pump light beams from multiple sources efficiently to achieve multi-kilowatt levels of high quality SM fiber laser output. Accordingly, there is a need for an efficient approach to couple enough high power diode pumps into a SM fiber laser to achieve multi-kilowatt levels of high quality SM fiber laser output.